In an electrophotographic apparatus employing the electrophotographic printing method, the developer is caused to adhere temporarily onto the surf ace of an image carrier, for example, a photoreceptor, on which an electrostatic latent image is formed in the developing step, transferred to a transfer sheet (copy sheet) from the image carrier in the transfer step, and then fixed to the transfer sheet in the fixing step.
As the developer for forming a copied image (toner image) by developing the electrostatic latent image, two-component developer composed of toner and carrier, and one-component developer (such as magnetic toner and nonmagnetic toner) requiring no carrier have been known.
Examples of the toner contained in the developer are positively charged toner and negatively charged toner. As additives for imparting a predetermined charging property to the positively charged toner, for example, charge control agents such as nigrosine compounds (dyes) and quaternary ammonium salt compounds are known. A known example of the additives for imparting a predetermined charging property to the carrier is a coating additive. Among these additives, the quaternary ammonium salt compounds are substantially colorless and can provide toners having a relatively large amount of charge.
Therefore, the quaternary ammonium salt compounds can be used not only for black toner, but also for color toner. Hence, in recent years, there is increasing demand for the quaternary ammonium salt compounds. For example, Japanese publication of unexamined patent application No. 76518/1996 (Tokukaihei 8-76518) discloses a toner to which a quaternary ammonium salt compound is added.
In general, a toner is produced as follows. First, raw materials including additives such as a binder resin, colorant, and charge control agent are mixed evenly. After melt-kneading the mixture, the mixture is ground and classified to provide the toner. External additives may be added to the toner, if necessary.
However, the above-mentioned conventional toner, i.e., toner containing the quaternary ammonium salt compound, can not retain an appropriate amount of charge, for example, when the electrophotographic apparatus is used continuously or when the toner is stored inside the electrophotographic apparatus for a long time. More specifically, for instance, when the electrophotographic apparatus is used continuously, the amount of charge tends to increase with an increase in the number of copies produced, and a lowering of the image density is a likely result.
Moreover, there is a significant difference in the charging property (charging characteristic) of toner between a normal atmosphere (for example, at a temperature of 25.degree. C. and relative humidity of 60%) and a high-temperature high-humidity atmosphere (for example, at a temperature of 35.degree. C. and relative humidity of 85%). Namely, the charging property of the toner is easily affected by the working atmosphere.
Thus, it is hard to say that the above-mentioned conventional toner can fully exhibit the effect (charge imparting effect) produced by the addition of the quaternary ammonium salt compound. In other words, since the conventional toner cannot retain an appropriate amount of charge irrespectively of the working atmosphere, the image density cannot be maintained in an appropriate level.
In addition, when transporting the toner, for example, in the case of domestic transport, the toner is sometimes kept loaded on the bed of a track parked for a long time under the blazing sun. In the case of transport to abroad, the toner is sometimes kept loaded in the non-air-conditioned cargo of a ship for a long time.
Like the above cases, if the conventional toner, i.e., the toner containing the quaternary ammonium salt compound, is left under the atmosphere of high temperatures exceeding, for example, 40.degree. C. for a long tine, the toner cannot retain an appropriate amount of charge.
Therefore, when such a toner left under the high-temperature atmosphere is used, a phenomenon (fog) in which the white portion of a transfer sheet to which a copied image is transferred overlaps the copied image occurs due to the vicious effect on the toner. As a result, the charging property (charging characteristic) is lowered, and the image quality is extremely degraded.
In the case when the copying machine (electrophotographic apparatus) is used continuously, the inside of the copying machine is made dirty by the toner. There is also a possibility that the atmosphere of the office and the like in which the copying machine is installed is worsened by the continuous use of the copying machine.
However, it is practically impossible to avoid a situation where the toner is left under the atmosphere of high temperatures exceeding, for example 40.degree. C. for a long time during transport of the toner. Hence, the charging property of the toner is easily affected by, for example, the transporting atmosphere.
It is thus hard to say that the above-mentioned conventional toner can fully exhibit the effect (charge imparting effect) produced by the addition of the quaternary ammonium salt compound. In other wards, since the conventional toner cannot retain an appropriate amount of charge irrespectively of the transporting atmosphere, the image density cannot be maintained at appropriate level.
Similarly, when the developer is left under the atmosphere of high temperatures exceeding, for example 40.degree. C. for a long time, the amount of charge decreases, causing a lowering of the image density. Namely, when the amount of charge of the developer decreases, the developer is not sufficiently supplied to the surface of the image carrier in the development step (copying). Consequently, the image density is lowered.
In such circumstances, there is demand for toner, developer and the process of producing toner, capable of overcoming the above-mentioned drawbacks.